Binary blends of polyetherimide and polycarbonate are known. Mellinger et al, U.S. Pat. No. 4,548,997 teach such blends as having a higher heat distortion temperature and improved flexural and tensile strengths over the polycarbonate alone. Additionally, such compositions have higher impact strength than the polyetherimide component and excellent flame resistance.
It is also known to blend polyetherimide with acrylate copolymers. Giles et al, U.S. Pat. No. 4,395,518 teach acrylate copolymer modified polyetherimides as having higher impact strength than is normally associated with the polyetherimide component as well as improved extrusion and molding performance characteristics.
More recently, Rock et al, Ser. No. 687,026 filed Dec. 28, 1984, have taught ternary blends of polyetherimide, polycarbonate and an acrylic rubber interpolymer having improved impact strength without any adverse effect on tensile and flexural properties or heat distortion characteristics as compared to the unmodified blend.
While all of the foregoing compositions have excellent impact strengths as compared to the polyetherimide component alone, as a result of the incorporation of polycarbonate and acrylic impact modifier, these compositions have increased flammability, whereas polyetherimides are known to have good flame resistance. Consequently, it is the purpose of the present invention to provide polyetherimide compositions having excellent physical characteristics, particularly impact strength, and good flame resistance.
Because polyetherimides are generally known to have excellent flame resistance in and of themselves, there has been no need for and thus no teaching or direction on how to improve the flame resistance of such compositions. While it is known in other polymer genera to employ various phosphorus and halogen containing compounds as a flame retardant additive, it is also well recognized that such flame retardants tend to be polymer or genus specific. That is, while it may enhance flame resistance in one polymer, it may have no effect in another. Consequently, the operation of flame retardants tends to be unpredictable. Furthermore, it is well recognized that the addition of such flame retarding compounds also adversely affects the physical characteristics of the polymer itself, often resulting in poor impact strength, reduced flexural and tensile strengths as well as reduced solvent resistance in some cases.
Therefore, there exists a need for a composition which exhibits excellent impact properties and flame resistance, without a concomitant loss in other desirable physical and chemical properties of the polyetherimide.